Return self in Operation.with_tags if adding no tags

[maffoo]

This is a tiny optimization that came up for me when trying to "copy" an operation with modifications. I want to be able to unconditionally add tags from the original operation but have it only create a TaggedOperation if needed, so I moved the logic to check whether the tags tuple is empty inside the with_tags method.

Also note that in most cases one does not have to check whether something isinstance(..., cirq.TaggedOperation), because we can access the .tags field unconditionally and it will simply be empty for untagged ops. Simplified the is_virtual_moment method in noise model to take advantage of this.